In a typical cellular radio system, also referred to as a wireless communication system, user equipments, also known as mobile terminals and/or wireless terminals, communicate via a Radio Access Network (RAN) to one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with the radio access network. The user equipment may also be referred to as a terminal or a UE.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNB”, “eNodeB”, “NodeB” or “B node” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g. by landlines or radio link, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
To support mobility of user equipments between cells, a cellular radio system must perform so called handovers. A handover is a change of serving cell, so that a user equipment being served by one cell becomes served by another cell instead. Handover may be necessary for example when a user equipment such as a mobile telephone moves from one serving cell into another, otherwise an ongoing call might be dropped due to loss of radio coverage. The decision of when a user equipment shall change to be served by another target cell is called a handover decision, and is often decided by the serving base station or another radio network node.
Handover decisions are based on measurement reports, provided to the cellular radio system by the user equipment itself, or by other base stations or nodes in the cellular radio system. For this purpose, user equipments may regularly perform measurements on signals originating both from the currently serving cell, and from surrounding cells. Each cell is identified by an individual so called identification code. The measured quantities may be for example received signal strength (RSS), signal to noise ratio (SNR), or bit error rate (BER). When the measurements indicate that a cell other than the currently serving cell would provide a better connection for a user equipment, a handover decision may be made.
To account for measurement uncertainties, and to avoid interruption of service, the cellular radio system typically has preset handover parameters to control handovers. Usually, before a handover decision is established, it is required that the measurements indicate that the measured quantity of the target cell is a certain amount better relative to the current serving cell, the so called “handover margin”, for a certain period of time, the so called “time to trigger”.
A tuning of handover parameters, such as the handover margin and the time to trigger, typically has an inherent tradeoff. Too small values might cause numerous switching between two cells, resulting in a user equipment being handed over back and forth from one cell to another. This may increase the system load and even if each handover only poses a minor risk for the connection due to signaling errors, the connection is exposed to an unnecessary risk if an excessive number of handovers is made. Conversely, too high values might also lead to a dropped call, since the handover decision might come too late.
To facilitate the handling of handovers, so called Neighbor Cell Relations (NCR) have been introduced.
In older wireless communication system revisions such as analog networks, GSM or UMTS, it is normally predetermined for each cell which neighbor cell relations can be established, whereas in newer wireless communication system revisions such as LTE—Long Term Evolution, a neighbor cell relation may be automatically established with any identified cell.
Adding to the difficulties in setting satisfactory handover parameters in a cellular radio system, is the fact that radio coverage of a cell is influenced by the surroundings, such as the topography of the terrain and buildings. Moreover, all base stations in a network may not be placed for optimum performance due to natural and enforced limitations. This results in most cellular systems having a “patchy” cell configuration with irregular cell borders and coverage islands of one cell inside another cell. This results in varying radio coverage conditions, for which the handover parameters may not always result in desired quality of handover service.
In an attempt to alleviate problems due to troublesome handovers, measurements may be performed manually, and some cell relations may be manually prohibited, or the handover parameters for a specific cell relation may be tuned to encourage or discourage handover.
Moreover, as networks become large, the number of cells and cell relations grow rapidly. The operators therefore demand a higher degree of automation, and so called Self Organizing Networks (SONs) are requested.
With a higher degree of automation in the wireless networks, neighbor cell relations might, as mentioned above, be added automatically by the network itself. Although this may be a desired property, it may also lead to neighbor cell relations being added that sometimes exhibit undesirable properties and lead to poor quality of service. The above stated problems associated with handovers may indeed be relevant for both older and newer wireless communication systems.